Hydraulic lift cylinder circuit



Jan. 8, 1963 J. E. oLsoN ETAL. 3,071,926

HYDRAULIC LIFT CYLINDER CIRCUIT Filed April l2, 1960 AT TORN E YS United States Patent Oiice 3,071,926 Patented Jan. 8, 1963 Filed Apr. 12, 1960, Ser. No. 21,626 1 Claim. (Cl. 60-52) This invention relates to industrial lift trucks and particularly to the hydraulic lift cylinder circuit of such a truck.

In recent years there has been an increasing demand for speeding up both the lifting and lowering movements of the load carriage of a lift truck. I'he capacity of the circuit pump of a truck has been a limiting factor in attempts to increase the lowering speed of the carriage because the lift piston cannot be allowed to descend at a speed that would cause cavitation above the piston, and cavitation will result if the speed is such that the pump is not able to supply uid at a rate to maintain the space above the piston full as the piston descends. This'critical lowering speed may be conveniently referred to as the pump limiting speed.

A main object of the present invention is to provide a lift truck having a hydraulic lift cylinder circuit which is operable to lower the carriage, without cavitation, at a speed exceeding the pump limiting speed.

Another object is to provide a simple hydraulic lift cylinder circuit which is automatically operable to elevate a light load at a faster rate than a heavy load.

A further object is to provide a hydraulic lift cylinder circuit having the features of both of the preceding objects.

Various other objects will be apparent from the following description taken in connection with the accompanying drawings wherein;

FIG. l is a schematic circuit diagram of a hydraulic system embodying the concepts of the present invention;

FIG. 2 is a fragmentary schematic diagram of a modified form of the invention;

FIG. 3 is a schematic `diagram of a further modiiied form of the invention.

FIG. l shows a conventional cylinder 11 having a piston 13 with a rod 15 projecting through the upper end of the cylinder 11 to raise a load directly or indirectly. In the conventional lift truck, the load is lifted indirectly by way of lift chains, not shown.

The circuit of the present invention includes a pump 21 connected by a line 23 to a manually operable carryover type directional control valve generally entitled 25, which in turn is connected to the lower end of the cylinder 11 by a line 27. The line contains a lowering control valve 26 which will be described presently.

The control valve 25 includes a shiftable spool which is manually shiftable to three different positions. In the first position, the neutral or hold position shown in FIG. l, the spool has a passage 31 to conduct fluid under pressure from the line 23 to a line 32 leading to a reservoir R. A line 33, to be mentioned hereinafter, is blocked at the valve in the FIG. l position.

The spool of the valve 25 is shiftable to the left to connect the lines 23 and 27 by means of a passage 37 so that fluid under pressure is conducted to the lower end of the cylinder 11 to raise the piston 13 therein. At such second spool position the lines 32 and 33 are blocked at the valve as indicated by the appropriate symbols. The spool is also shiftable to the right to place lines 27 and 33 in communication with one another by means of a passage 39, and to connect lines 23 and 32 by means of a passage 40.

A line 41 connects the line 27 to a line 42, but a check valve 43 prevents uid under pressure from passing upwardly through the line as the parts are shown in FIG. l. The line 42 is connected to a snap action sequence valve 49 which is pilot operated by pressure in a line 51 connected to the line 27. The sequence valve 49 is connected by a line 53 to a maximum pressure valve 55 which is connected by a line 57 to the reservoir R. The line 33, previously mentioned, is connected to the line 53.

There is no convenient way to show completely a common sequence valve in the J.I.C. Code, but its construction is Vwell known. The spring-pressed-movable valve member of the valve has a small area portion closing a small opening. When the member is unseated by a suflicient pressure, a large area of the member is subjected to fluid pressure. Thus, once the member is unseated, a much lower pressure can hold it in its unseated position than was required to unseat the valve. For instance, in atypical installation of the present invention, a sequence valve was employed that opened under 1500 pounds per square inch of pressure, and remained open until the pressure dropped to 100 pounds.

Referring to FIG. 1, the position of the line 61 of the valve 49 indicates that the valve is normally closed and urged to so remain by a spring 63. If the pressure in the pilot line 51 is of sufficient magnitude, it can unseat or open the valve. A `check valve 65 is shown diagrammatically within the sequence valve to indicate that opening of the valve by pressure in line 51 opens the check valve. In practice, the movable valve member of the valve 49 engages and unseats the check valve, which is separate from but located adjacent to the sequence valve.

When the check valve is not unseated by the pressure in the line 51, it still allows movement of iiuid from line 53 to line 42 but prohibits flow iu the opposite direction.

The maximum pressure valve 55 has a spring-biased valve element 69 which is opened by pressure in the line 53. In a typical installation, the valve 55 will open at 100 pounds pressure to maintain a pressure of approximately l0() pounds per square inch in lines 42 and 53.

Operation Assume that the load is substantial, say 2,500 pounds. To Ilift the load, the operator will shift the spool of the valve 25 to the left to connect the pump to the bottom of the cylinder 11. The valve 26 has a check valve 70 which permits flow toward the cylinder, and a restriction 71 permits ow away from the cylinder at a desired rate to prevent the load from dropping at too high a rate. The substantial fluid pressure below the piston 13 necessary to lift the load will be effective through the pilot line 51 to open the sequence valve 49 to allow uid from the top of the cylinder to ow through the line 42 and the sequence valve and thence through the maximum pressure valve 55 to the reservoir R. The load is thus lifted at a rate determined by the rate of supply of uid from the pump 21.

To lower the load, the spool of the valve 25 is shifted to the right to centrally locate the passage 39. Fluid below the piston 13 can flow through the lines 27, 33 and 53 to the valve 49 and through the valve to the upper end of the cylinder. The position of the valve 49 at this time is immaterial, since in either of its positions iluid could flow upwardly from line 53 to the line 42.

The provision of only one piston rod 15 for the piston 13 means that there is a greater volume of fluid below the piston per unit length of the cylinder than above the piston. Thus, when the piston moves downwardly and the uid exhausted from the lower end of the cylinder is conducted to the upper end of the cylinder, there will be surplus or excess lluid. This excess uid is forced downwardly through the line 53, and when the pressure buildup is suicient, it will open the valve S. This means that uid in the lines 33, 53 and 42 is under a pressure established by the valve 55, and thus the fluid delivered to the upper end of the cylinder is under a positive pressure.

From the above discussion, it is apparent that the load is lowered rapidly at a rate independent of the supply of the lluid from the pump 21 and without cavitation in the upper end of the cylinder. The restriction 71 and the pressure necessary to open the valve 55 determine the lowering rate, and it is contemplated that the restriction and valve will be chosen so that the lowering rate exceeds the pump limiting speed of the carriage.

Now assume that the load to be lifted is a light load, say 500 pounds. The operator again shifts the spool of the valve 25 to the left to supply lluid under pressure to the lower end of the cylinder. However, the uid pressure necessary to lift the light load is not suicient to operate the sequence valve 49 so that it remains in its FIG. l position. The uid pressure above the piston 13 thus builds up until it opens the check valve 43 and llows into the lower end of the cylinder. This fluid thus augments the flow from the pump 21. Under these circumstances, the eiective area of the piston is the area of the rod 15, and thus the piston 13 is lifted at a rapid rate, considerably faster than when lifting a heavier load.

The light load is lowered in the same manner as a heavy load and at approximately the same speed.

Modied Forms The circuit of FIG. 2 differs from that of FIG. 1 in that a valve 78 is provided in the line 51a to enable the valve 49a to be connected to the line 41a when the valve 7S is positioned as shown, or connected to the line 23a by a line 79 when the valve 78 is turned to the proper position.

The advantage of the FIG. 2 circuit is that it will maintain the versatile operation of the circuit despite variations in the weight of the carriage such as might result by changing from a fork carriage to a revolving clamp. For instance, assume that the valve 49a is connected to the line 41a, and that a heavy carriage is utilized. If a heavy load is deposited at a high level and the carriage lowered to an intermediate level to pick up a light load, which is to be then elevated to said high level, the weight of the heavy carriage alone creates too high a pressure to allow the sequence valve 49a to shift back to its closed position after the carriage has been relieved of the heavy load. Thus the light load would be lifted at the regular speed of the carriage rather than at high speed. Of course, the valve 49a can be chosen so that it will shift back despite the use of a heavy carriage, but for any one valve 49a, the valve 78 facilitates high speed lift of the carriage under the above circumstances even though the carriage is changed to a weight above that for which the valve 49a is designed. The valve 78 does this by enabling the valve 49a to be connected to the line 23a. Thus, when the empty heavy carriage is lowered from a high level to an intermediate level, the valve 25a connects the line 23a to the reservoir R and thus the valve 49a will shift to its closed position so that a subsequent elevation of the carriage with a light load will be at high speed.

The FIG. 3 circuit is somewhat similar to the FIG. 1 circuit but differs in that (l) the control valve 25h is different from the valve 2S; (2) a self-operated pressure reducing valve 81 is connected by lines 83 and 85 in parallel with the check valve 43h; (3) line 33 is eliminated; and (4) the valve 49b does not have a check valve 65. The pressure reducing valve 81 opens at a low pressure in the line 8S and closes at a higher pressure. The higher pressure, however, is lower than the pressure set by the valve 55. For instance, the valve 81 may be set to open at pressures below 50 pounds per square inch.

The valve 25b is a two-port valve instead of a threeport valve, and specically differs from the valve 25 because of the elimination of the line 33. This elimination means that passages 39 and 40 of FIG. 1 are combined into an h shaped passage 39b in FIG. 3 which connects the lines 23b and 27b to the reservoir R when the valve 25b is shifted to the right.

YIn lifting a heavy load, sequence valve 4% is opened by the high pressure line 27b and thus the back pressure in line 42b is reduced and fluid from the rod side of the cylinder can llow to the tank directly instead of through the regenerative circuit to be described later.

When raising a light load, the pressure in line 27b is insuflicient to open valve 49:'7 so the oil from the rod side of the cylinder is conducted to the piston side of the cylinder thus supplementing the output of the pump and increasing the speed of lift.

When lowering either a heavy load or a light load, control of the speed at anything less than the safety `limit determined by restriction 7119 is controlled by feathering valve 25b.

During lowering, after the piston starts down, the pressure in line 4211 tends to be reduced which would cause cavitation were it not for the fact that pressure reducing valve 81 will open suiliciently to allow fluid to pass from line 83, through valve 81 and thence to the top of the cylinder in sufficient quantity to maintain a positive pressure in the rod end of the cylinder.

In view of the fact that valve 49b is sometimes open (depending on whether a light or heavy load is being carried) it is necessary to back up the positive pressure maintained in lines 85 and 42h by valve 81 or the oil would ilow to the tank instead of to the top of the cylinder. This back-up pressure is provided by valve h which will not open until the pressure in line 5311 reaches a value above the positive pressure maintained during lowering in lines 85 and 2b.

The important point here is that `lluid is allowed to pass from the piston end to the rod end of the cylinder in a regenerative cycle which makes it possible for the load to be lowered at a speed greater than the capacity the pump may have to lill the rod end of the cylinder. It is to be noted also that the iluid required to lill the rod end of the cylinder does not ever have to pass through control valve 25b. This is important because all control valves offer a considerable resistance to the flow of lluid.

Having described the invention in what is considered to be the preferred embodiment thereof, it is desired that it lbe understood that the invention is not to be limited other than by the provisions of the following claims.

We claim:

A hydraulic lift cylinder circuit for a lift truck comprising a vertical cylinder, a piston in said cylinder having a rod projecting through the upper end of the cylinder, a sump, a maximum pressure valve connected lto said sump, a control valve, a positive displacement pump connected to said control valve, lluid passageway means connecting said valve to the lower end of said cylinder, said control valve having three positions, one in which said pump is placed in communication with said sump, a second in which said pump is placed in communication with said passageway means, and a third in which said passageway means is placed in communication with said maximum pressure valve and said pump is placed in communication with said sump, a normally closed valve connected between said maximum pressure valve and the upper end of 5 6 said cylinder and responsive to a predetermined pressure and check valve means lay-passing said restriction means in said passageway means to be actuated to an open po-siand allowing free ilow of fluid from said control Valve to tion, said passageway means constituting the sole source the lower end of said cylinder. of the actuating pressure for said normally closed valve, l check valve means for allowing ow of uid from the di- 5 References Cled H1 the le 0f thlS Patent rection of said maximum pressure Valve to the upper end UNITED STATES PATENTS of said cylinder, check valve means for allowing the flow of fluid from the upper end of said cylinder to the lower 2 'Sreng D213 end, restrictlon means for restrictmg the llow of -lluld 2,499,425 Stephens Mar. 7, 1950 from the lower end of said cylinder to vsaid control valve, 10 

